This application addresses broad Challenge area (04) Clinical research, and specific Challenge Topic 04-HL-103 Assess the role of leukocyte interaction with platelets, erythrocytes, and endothelium in the pathogenesis of heart, lung, and blood diseases. Cell migration plays essential roles in embryonic development and inflammation, and this process is highly regulated to ensure tissue homeostasis. The migration of blood cells and their interactions with the cells of the vasculature, have been studied for over 100 years, from microscopic examination of simple organisms, through complex biochemical and genetic studies, to high resolution imaging of immune cells in during inflammation in living mammals. These studies have provided a great understanding of the mechanisms of inflammation and have allowed the development of new therapeutics for inflammatory and immune disorders. However our knowledge of the mechanisms by which tissue homeostasis is maintained and whether immune cells are actively excluded from non-inflamed tissues in the absence of inflammation is incomplete. Similarly, we have little understanding of the signals that can drive leukocyte exit during resolution of inflammation and whether this process is defective in chronic inflammatory conditions. Recent work from our lab and others has led us to hypothesize that 'negative guidance cues'which inactivate cell migration or mediate cell repulsion, such as those described in the developing nervous system, are also employed by endothelial cells to impede inappropriate recruitment and/or accumulation of immune cells in tissues. In our preliminary data we have found that representative guidance molecules inhibit leukocyte migration in vitro, are expressed by non-inflamed endothelial cells and prevent leukocyte attachment and recruitment to tissues in vivo. However, under cell stress or following exposure to inflammatory cytokines, expression is reduced, lowering these barriers for inflammatory cell recruitment. We believe that these data represent a new paradigm for leukocyte: endothelial interactions. In this grant application, we propose to establish the role of these guidance molecules in leukocyte: endothelial cell interactions, and identify guidance molecule: receptor pairs that mediate repulsive and attractive interactions. These studies will increase our understanding of the mechanisms that lead to recruitment, retention, and exit of inflammatory cells in the vasculature and other tissues in and will provide potential therapeutic targets for modulation of leukocyte accumulation. PUBLIC HEALTH RELEVANCE: Cell migration plays essential roles in embryonic development and inflammation, and this process is highly regulated to ensure tissue homeostasis. In this challenge grant, we propose to establish a new paradigm for inflammatory cell trafficking, identifying neuronal guidance molecule: receptor pairs that mediate repulsive and attractive interactions of leukocytes with vascular endothelium.